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Сентябрь 6, 2022

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2. FREE ENERGY AND EQUILIBRIA ΔG ΔG > 0 reverse process is spontaneous ΔG = 0 no
3. CHEMICAL POTENTIAL The chemical potential can be used to give quantitative meaning to Le Châtelier’s principle.
4. DIRECTIONALITY OF A CHEMICAL REACTION Consider a closed system of four components (A, B, C, and
5. EXAMPLE H2O (l) H2O (g) 100 °C This is at constant T and P. Its reversible.
6. CHEMICAL POTENTIAL AND PARTIAL PRESSURE We found last time that: (T = constant) Defining the standard
7. ΔG OF MIXING Consider the isobaric, isothermal mixing of two gases: Gas A at P Atm
8. EQUILIBRIUM CONSTANT The Haber process of nitrogen fixation: N2 + 3H2 2NH3 Rewrite letting unitless pressure
9. EQUILIBRIUM CONSTANT At any temperature and pressure there exists and equilibrium state for this reaction. A
10. EQUILIBRIUM CONSTANT Define equilibrium constant (at constant T and P): More generally: aA + bB cD
11. CHEMICAL POTENTIAL EXAMPLE Reactants Products Reactants Products Δμ° Δμ° μ° Chemical potential is a measure of
12. EQUILIBRIUM EXAMPLE Oxidation of CO: 2 CO (g) + O2 (g) 2 CO2 (g) The free
13. EQUILIBRIUM EXAMPLE Reactants (CO,O2) Products (CO2) Δμ°, ΔG° The conversion of CO and O2 to CO2
14. ΔH0 FOR SOME NONCOVALENT INTERACTIONS Na+(g) + Cl-(g) NaCl(s) Ionic -785 NaCl(s) Na+(aq) + Cl-(aq) Ionic
15. PROTEIN UNFOLDING Proteins have a native state. (Really, they tend to have a tight cluster of
16. PROTEIN UNFOLDING Let’s consider denaturation with heat. We can determine a great deal about the nature
17. PROTEIN UNFOLDING In differential scanning calorimetry you have two samples: Your material of interest Control You
18. PROTEIN UNFOLDING Data for glyceraldehyde-3-phosphate dehydrogenase. pH8.0 pH6.0 Bacillus stearothermophilus E. coli Rabbit Is the protein
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Слайд 2

FREE ENERGY AND EQUILIBRIA
ΔG < 0 process is spontaneous
ΔG > 0 reverse

process is spontaneous
ΔG = 0 no change is spontaneous: Equilibrium
Le Châtelier’s principle gives us some understanding of how the equilibrium of a system changes when we perturb it.
We are interested in how systems tend towards equilibrium and what obstacles keep them out of equilibrium.
The free energy, or more correctly, the chemical potential is our measure of roughly how far we have to go to come to equilibrium. It is a potential energy of sorts.

"Any change in one of the variables that determines the state of a system in equilibrium causes a shift in the position of equilibrium in a direction that tends to counteract the change in the variable under consideration."

Слайд 3

CHEMICAL POTENTIAL
The chemical potential can be used to give quantitative meaning

to Le Châtelier’s principle.
Chemical potential of component A, µA, is defined as the partial molar Gibbs free energy:
This is the change in G with respect to a infinitesimal change in the amount of component A with all other parameters held constant.
It is essentially the free energy increase (or decrease) associated with adding a little of A to the system.

Слайд 4

DIRECTIONALITY OF A CHEMICAL REACTION
Consider a closed system of four components

(A, B, C, and D) undergoing a reversible chemical reaction:
In a closed system,
At constant T and P,
At equilibrium:

Слайд 5

EXAMPLE
H2O (l) H2O (g)
100 °C
This is at constant T and P. Its

reversible. So,
ΔG°vap= 0 = ΔH°vap-T ΔS°vap
Note that this implies at equilibrium:

Слайд 6

CHEMICAL POTENTIAL AND PARTIAL PRESSURE
We found last time that: (T =

constant)
Defining the standard state:
Then,
and,
so,
In a mixture, the chemical potential of A can thus be expressed in terms of its partial pressure and its standard (pure) chemical potential:

Слайд 7

ΔG OF MIXING
Consider the isobaric, isothermal mixing of two gases:
Gas A
at

P Atm

Gas B
at
P Atm

Gas A+B
at
P Atm (total)

Mole Fraction component i

Слайд 8

EQUILIBRIUM CONSTANT
The Haber process of nitrogen fixation: N2 + 3H2 2NH3
Rewrite

letting unitless pressure ratio. No units inside “ln”.

We have used the identity:

Слайд 9

EQUILIBRIUM CONSTANT
At any temperature and pressure there exists and equilibrium state

for this reaction.
A combination of such that ΔG = 0.

This equation holds for equilibrium values of

Слайд 10

EQUILIBRIUM CONSTANT
Define equilibrium constant (at constant T and P):
More generally: aA +

bB cD + dD

Most generally:

Слайд 11

CHEMICAL POTENTIAL EXAMPLE
Reactants
Products
Reactants
Products
Δμ°
Δμ°
μ°
Chemical potential is a measure of the thermodynamic free

energy.
It tells us what the equilibrium distribution of reactants and products must be
It does not tell us the kinetic rate.

Activation barrier

Слайд 12

EQUILIBRIUM EXAMPLE
Oxidation of CO: 2 CO (g) + O2 (g) 2 CO2

(g)
The free energy change for this reaction is simply:
ΔG ° rxn= 2 ΔG°289(CO2) - 2 ΔG°289(CO) - ΔG°289(O2)
We can calculate this using numbers from the appendix:
ΔG ° rxn= 2 (-394.36) - 2( -137.17) - 0 (kJ/mol) = -514.38 kJ/mol
A relatively large negative number.
ΔG ° rxn= -RTlnK K = e514,380/RT= e207 =
The equilibrium for this reaction lies far in favor of the products.
Large negative ΔG means the reaction goes forward with high probability.

Слайд 13

EQUILIBRIUM EXAMPLE
Reactants
(CO,O2)
Products (CO2)
Δμ°, ΔG°
The conversion of CO and O2 to CO2

is
energetically favorable, but the reaction is slow
1) We have to first break an O-O double bond
2) The resultant atoms of oxygen must
then react with CO.

For other reactions like
2 H2(g) + O2(g) 2 H2O (l)
We need a catalyst (e.g. platinum surface), and ideally we wish to convert the released energy to work.
Catalysts enable a reaction to take an alternative path, with lower activation barrier.

Reaction coordinate

Слайд 14

ΔH0 FOR SOME NONCOVALENT INTERACTIONS
Na+(g) + Cl-(g) NaCl(s) Ionic -785
NaCl(s) Na+(aq) +

Cl-(aq) Ionic +ion-dipole +4
Ar(g) Ar(s) London (fluctuation dipole) -8
Acetone(g) Acetone(l) London-van der Waals -30
H3C H3C (permanent dipole)
2 O O H O Hydrogen bond -20
H H CH3
H
H2NON H
H OH2 H2NON Hydrogen bond (aq) -5
H2NONH2 H O(NH2)2
HOH (Urea (aq))
C3H6(l) + H2O(l) C3H6(aq) Hydrophobic -10
(TSWP p. 98)

Reaction Interaction ΔH0(kJ mol-1)

Слайд 15

PROTEIN UNFOLDING
Proteins have a native state. (Really, they tend to have

a tight cluster of native states.)
Denaturation occurs when heat or denaturants such as guanidine, urea or detergent are added to solution. Also, the pH can affect folding.
When performing a denaturation process non-covalent interactions are broken.
Ionic, van der-Waals, dipolar, hydrogen bonding, etc.
Solvent is reorganized.

Слайд 16

PROTEIN UNFOLDING
Let’s consider denaturation with heat. We can determine a great

deal about the nature of the protein from such a consideration.
The experimental technique we use for measuring thermodynamic changes here is the differential scanning calorimeter.
Basic experiment: Add heat to sample, measure its temperature change.

heat

Слайд 17

PROTEIN UNFOLDING
In differential scanning calorimetry you have two samples:
Your material

of interest
Control
You put in an amount of heat to raise the temperature of the control at a constant rate, then measure the rate of change in temperature of the other sample as a function of the input heat.
This is a measure of the heat capacity!

Protein
+
Solvent

Solvent

Heat

T1-T2

Слайд 18

PROTEIN UNFOLDING
Data for glyceraldehyde-3-phosphate dehydrogenase.
pH8.0
pH6.0
Bacillus stearothermophilus
E. coli
Rabbit
Is the protein more stable

at pH 8 or 6? Why is B. stear. more stable?

физхим асан

Содержание

FREE ENERGY AND EQUILIBRIAΔG < 0 process is spontaneousΔG > 0 reverse

CHEMICAL POTENTIALThe chemical potential can be used to give quantitative meaning

DIRECTIONALITY OF A CHEMICAL REACTIONConsider a closed system of four components

EXAMPLEH2O (l) H2O (g)100 °CThis is at constant T and P. Its

CHEMICAL POTENTIAL AND PARTIAL PRESSUREWe found last time that: (T =

ΔG OF MIXINGConsider the isobaric, isothermal mixing of two gases:Gas Aat

EQUILIBRIUM CONSTANTThe Haber process of nitrogen fixation: N2 + 3H2 2NH3Rewrite

EQUILIBRIUM CONSTANTAt any temperature and pressure there exists and equilibrium state

EQUILIBRIUM CONSTANTDefine equilibrium constant (at constant T and P):More generally: aA +

CHEMICAL POTENTIAL EXAMPLEReactantsProductsReactantsProductsΔμ°Δμ°μ°Chemical potential is a measure of the thermodynamic free

EQUILIBRIUM EXAMPLEOxidation of CO: 2 CO (g) + O2 (g) 2 CO2

EQUILIBRIUM EXAMPLEReactants(CO,O2)Products (CO2)Δμ°, ΔG°The conversion of CO and O2 to CO2

ΔH0 FOR SOME NONCOVALENT INTERACTIONSNa+(g) + Cl-(g) NaCl(s) Ionic -785NaCl(s) Na+(aq) +

PROTEIN UNFOLDINGProteins have a native state. (Really, they tend to have

PROTEIN UNFOLDINGLet’s consider denaturation with heat. We can determine a great

PROTEIN UNFOLDINGIn differential scanning calorimetry you have two samples: Your material

PROTEIN UNFOLDINGData for glyceraldehyde-3-phosphate dehydrogenase.pH8.0pH6.0Bacillus stearothermophilusE. coliRabbitIs the protein more stable

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